Search "leveling commands" in Kingdom Hearts: Birth by Sleep Tags :

Secret - Leveling Commands with Terra

A good place to level commands is Peter Pan's World. Follow these steps to get your commands from 1 to master.
1-Start in the cove and head over to cliff path right next the safe spot. Put on the commands that need leveling before you start fighting.

2-Start fighting and and make your way to mermaid lagoon. There you'll see more unversed to fight, but don't fall to the water.

3-After you're done go back to cliff path and defeat more unversed there.

4-Continue to the cove to save the game if you need and to change the commands you already mastered.

Doing these steps you'll be leveling up commands, shot locks, finish commands and even your own level. You get about 1477 XP every time you start from cove and get back (not including having the EXP Walker and EXP Chance ability on.)